To provide a method for determining the acceptance/rejection of the weldability without measuring the amount of gas in a die-cast product, and controlling the weldability of the die-cast product at a casting site.

A test piece formed in a vicinity of a weld part in a die-cast product is cut off, the specific gravity of the test piece before and after the heat treatment is measured, and the swelling ratio of the test piece is calculated by the formula: (specific gravity before heat treatment - specific gravity after heat treatment) / specific gravity after heat treatment × 100 based on the measured value. If the calculated swelling ratio exceeds the reference value, it is determined that the amount of gas in the weld part is less, and the weldability is excellent. If the swelling ratio exceeds the reference value, it is determined that the amount of gas in the weld part is more, and the weldability is poor.